This invention relates generally to power line communications systems. In a particular embodiment, it relates to a power line communication system for use in communicating through a distribution transformer.
Power line or xe2x80x9ccarrier-currentxe2x80x9d communication systems employ existing alternating current power lines to transfer information which would normally require an additional hard wire installation. Power line communication systems are well-known and extensively used. However, power line communication systems which are capable of communicating through a distribution transformer must overcome both the attenuation of high carrier frequencies due to the impedance of the distribution transformer and the noise on power lines at lower frequencies.
Power line communication systems that have the capability of sending data along a signal path that includes distribution transformers are known. However, the methods by which these systems have addressed the twin problems of attenuation of high carrier frequency data signals by the distribution transformer and noise on the power line at lower frequencies have disadvantages.
Where lower carrier frequencies are used, the transmitters tend to be expensive, bulky and power hungry and require special installation. In other cases where lower carrier frequencies are used, the transmitter may be small, but the receiver is bulky and expensive and the system is not capable of sending and receiving data at a useful rate.
Where known power line communication systems use high carrier frequencies for communication along a signal path that includes distribution transformers, they require additional elements beyond the transmitter and receiver in order to overcome the attenuation problems associated with communicating through the distribution transformer. As a result, these systems cannot be installed by simply plugging a transceiver into a typical wall outlet.
One earlier proposal that addresses the problem of attenuation when communicating through a power distribution transformer using a high carrier frequency is found in U.S. Pat. No. 4,406,249 issued to Pettus. U.S. Pat. No. 4,406,249 suffers from the disadvantage that coupling capacitors must be installed in common mode such that the carrier frequency signals can be introduced on both secondary leads. Common mode coupling requires access to both secondary leads from the distribution transformer and a typical wall outlet only allows access to one of these leads. Therefore, a transceiver according to this patent could not be installed by simply plugging it into a wall outlet using this system. Also, the high carrier frequency of 230 kHz which Pettus teaches is high enough to cause radio interference and produce signal node points along the power line where the signal is too weak to be picked up.
Another earlier proposal that addresses the problem of attenuation when using a high carrier frequency to communicate over power lines where the signal path includes distribution transformers, is found in U.S. Pat. No. 4,142,178 issued to Whyte. U.S. Pat. No. 4,142,178 does not teach a method of communicating through a distribution transformer, rather it teaches a high voltage signal coupler which is used to couple a high voltage distribution network primary conductor to the communication elements. This method bypasses the distribution transformers. While this alleviates the problem of attenuation, as the communication system is not connected to the secondary of the distribution transformers it cannot be installed by simply plugging the transceivers into wall outlets. Also, the coupling system requires installation of magnetic cores on the high voltage and ground conductors of the power line system which is relatively complex and adds cost. Frequency shift keying (FSK) is a known power line communication technique wherein the transmitter modulates a reference frequency signal based upon the data to be transmitted, so that the transmitted signal has a frequency which is either higher or lower depending on whether a logic 1 or a logic 0 is being transmitted. The receiver is designed to demodulate the transmitted FSK signal to produce a serial data stream at a predetermined rate (baud rate). In order to generate an accurate reference frequency, the nominally 60 Hz power-line frequency can be used. An earlier example of the use of the power line frequency as a reference frequency for generating the carrier frequency in an FSK system can be found in U.S. Pat. No. 4,556,866 issued to Gorecki. However, U.S. Pat. No. 4,556,866 suffers from the disadvantage of using a phase locked loop in conjunction with the power line frequency in order to generate the reference frequency. Phase locked loops are a source of noise, are prone to instability, and are sensitive to component values which can change with temperature and age. These disadvantages make it undesirable to use a phase locked loop in a FSK transmitter designed to communicate through a distribution transformer.
Known power line communication system transmitters use amplifier circuits in which the design is optimized for parameters not related to energy efficiency. The presence of energy losses results in heat dissipation, which requires additional energy for producing the signal, but more significantly results in a larger physical size being required for the transmitter. This is needed to provide the extra surface area required to remove the heat without high temperatures developing which could cause failure of the device. It is known that efficiency of the amplifier can be improved through the use of switch mode amplifiers instead of the more common but less efficient linear amplifiers, but even such devices will not achieve the best efficiency for a power line signal transmitter if they are optimized for parameters that are not relevant to this purpose.
It is also known to use a resonant coupling network which is xe2x80x9ctunedxe2x80x9d to resonate at the carrier frequency to couple the carrier frequency signal to the power line in order to increase signal level. The use of a resonant or xe2x80x9ctunedxe2x80x9d circuit to boost the transmitter signal is common. A resonant circuit comprises one or more inductors and one or more capacitors either in series or in parallel so that energy is transferred back and forth between the inductors and capacitors in a cyclic manner at the power line carrier frequency, in a manner that is analogous to the way a weight bobs up and down when suspended by a spring. In the simple case where there is only one inductor and one capacitor, electrical resonance is achieved by selecting the inductor and the capacitor such that:   f  =      d          2      ⁢              xe2x80x83            ⁢      π      ⁢              LC            
where:
f=power line carrier frequency in cycles/second
xcfx80=3.14159265358979 . . .
L=inductance in Henrys
C=capacitance in Farads
d=duty cycle (between 0 and 1)
Normally xe2x80x9cdxe2x80x9d is set to 1 unless the resonant network is switched.
When a resonant circuit is used to couple the carrier frequency signal onto the power line, an increase in transmitter efficiency is achieved. In some cases, energy efficiency may still be low, particularly if the network is primarily designed to attenuate unwanted frequencies. Also, the known techniques of producing resonance also tend to introduce energy losses which substantially reduce efficiency improvements. Sometimes these losses occur in resistive elements added to the resonant circuit. In other cases, the losses occur because the amplifiers or transistors, which drive the resonant network, are running in a linear mode. In this mode, they are neither completely off nor completely on. Therefore, they dissipate heat the way resistors do. Sometimes, the losses occur because of large current surges through semiconductor components, which result when capacitors are suddenly charged up or discharged. Even though these surges may be very brief, they can cause much heating since heating is proportional to the square of the current level. While the cost of the energy may not be a problem, heat dissipation, particularly in semiconductors increases the need for heat dissipation material which adds to the size weight and cost of the transmitter.
Examples of proposals using resonant circuits to improve transmitter efficiencies in power line communication systems can be found in U.S. Pat. No. 4,142,178, U.S. Pat. No. 4,323,882, U.S. Pat. No. 4,517,548, U.S. Pat. No. 4,636,771, U.S. Pat. No. 4,885,563, U.S. Pat. 5,485,040, and U.S. Pat. No. 5,870,016. However, each one of these references suffers from one or more of the disadvantages described in the preceding paragraph.
Efficiency is also an issue in the receiver circuit used in a power line communication system. The use of analog circuit methods in receivers is known. A disadvantage of analog circuit methods is that frequency pass bands are set by networks of reactive components whose values may change due to temperature and other factors. Unintended changes in component values can result in a loss of signal.
It is also known to use digital methods to process the received signals. The use of quartz crystal oscillator based digital frequency synthesis allows the frequency pass bands to be set much more accurately. However, the accuracy is still limited by the accuracy of the quartz crystal oscillator which also may be affected by temperature and other factors. Digital signal processing methods may also suffer from digitization errors, particularly if the signal level is small in comparison with the voltage resolution of the analog-to-digital converter that is used. The effect of this is to increase the need for averaging, thereby reducing the data rate achievable.
Accordingly, there is a need of a power line data communication system where both the transmitter and receiver are small in size and weight, and where both are easily installed, simply by plugging into an ordinary electrical outlet without the need for repeaters, coupling networks or additional equipment, and where data signals can sent and received at useful rates through distribution transformers.
One advantage of such a communication scheme would be to facilitate transfer of routine and relatively small quantities of date to individual customers or electrical utility, including both residential and industrial customers of an electrical utility, including both residential and industrial customers. Indeed, one expected usage is reading of electricity meters, to enable recording of the amount of electricity used and generation of bills. Currently, reading of meters has to be done manually, which is time consuming and expensive, and if, for example, a residential or other customer is not available, the meter may not be at an external location for reading.
It is also possible that such scheme could be used to read a variety of other utility meters, for example, gas and water meters. While other widespread telecommunication networks are known, using power lines has some advantages. The infrastructure is inherently in place. If a system is set up to bill customers on a periodic basis, e.g. monthly, then it is a simple matter to extend this to gathering additional data from other meters at the same location.
The quantity of data, to be transmitted is, by telecommunication standards, low, so that it is possible to consider collecting and transmitting data for various devices together.
It is therefore desirable to provide a power line communications system for use in communication of data over power lines, including through a distribution transformer and capable of being installed by plugging into a wall outlet, said power line communications system comprising a transmitter having a pair of terminals for connection to the power lines. The transmitter comprises a carrier frequency generator for generating a carrier frequency modulated by the data signal and a switching circuit connected to the carrier frequency generator for being switched by the carrier frequency generator to generate a carrier signal having the carrier frequency. The switching circuit is connected to the terminals for providing the generated carrier signal thereto. The switching circuit comprises at least one storage means for storing energy when generating a portion of a cycle of the carrier signal and providing the stored energy when generating another portion of the cycle of the carrier signal. The system also comprises a receiver coupled to the power lines. The receiver includes a filter means for filtering the carrier signal from the power line signal and a demodulator connected to the filter means for extracting the data signal from the carrier signal.
In a further aspect, the present invention may also comprise a carrier frequency generator for continuously calculating a carrier frequency as a function of the power-line frequency for adaptively tracking changes in the power-line frequency. The carrier frequency generator has a synchronization input for connection to the power lines and the transmitter generates a carrier signal having the carrier frequency and provides the carrier signal. The system further comprises a receiver for connection to the power lines. The receiver comprises a filter means for filtering the carrier signal from the power signal, a demodulator for extracting the data signal from the carrier signal, and a synchronization input for connection to the power lines. The receiver continuously generates a carrier decode frequency as a function of the power-line frequency for adaptively tracking changes in the power-line frequency and for demodulating the carrier signal. Both the demodulator and carrier frequency generator utilize a digital algorithm for calculating the carrier frequency and the carrier decode frequency thereby providing frequency alignment between the transmitter and the receiver.